Mechanical separation of a blade from its rotor while a rotating machine is operating can cause significant damage to the machine and may potentially be hazardous to personnel. Moreover, cracks that may occur in blades can grow to a critical length during operation of the machine and may lead to separation of the blade resulting in damage to the machine.
It is desirable to reduce the likelihood of blade separation. For that purpose, it is a common practice to perform periodic non-destructive examinations of blades in rotating machines. Methods that have been used for such non-destructive examination include visual inspection, magnetic particle inspection, fluorescent penetrant inspection, eddy current inspection, ultrasonic phased-array inspection, and acoustic thermography inspection. Conventional application of these non-destructive examination techniques requires that the turbine rotor be stationary during the inspection.
Even if a crack is not detected during such a periodic stationary non-destructive examination, it is potentially possible for such a crack to initiate and grow to critical size between such examinations. To address this possibility, online systems and methods are known for monitoring the blades while the machine is operating, such as described in U.S. Pat. No. 7,432,505 titled “Infrared-based Method and Apparatus for Online Detection of Cracks in Steam Turbine Components.” In this manner, analysis and decision systems may be employed to summarize data and make decisions regarding the operation of a rotating machine such as a turbine.
One potential approach to online monitoring is based on the observation that the presence of a crack in a blade can change the stiffness and therefore the natural frequencies of that blade. Methods are known by which the vibration amplitudes of a blade can be measured as a function of time using, for example, strain gages or tip timing measurements. Such amplitude measurements can be transformed from the time domain into the frequency domain using, for example, FFT techniques. Such techniques have been applied to analyze data for rows of blades, considering each individual blade successively.
Methods have been proposed to use temporal changes in such assessed blade frequencies to detect blade cracks. Such methods have been found not to be robust. For many useful applications, the variations in the frequencies assessed for the same row of uncracked blades at different times and different operating conditions have been found to be of the same order as the changes in frequencies that would result from the presence of a crack of significant size.
The variations in the assessed frequencies arise from the complexity of vibratory behavior that can exist even in a theoretical row of identical blades. For a single blade, each basic mode of vibration, such as for example the fundamental flex bending mode, has a single associated eigenfrequency. But a row of “n” such blades, there are “n” such eigenfrequencies for each basic mode of vibration, each such eigenfrequency being associated with a different nodal diameter.
In real blade rows, the vibratory characteristics of each individual blade are not identical. Understanding the behavior of this type of complex blade system is an active area of research that is referred to in the literature as mistuning.
Accordingly, there is a need in the art for an improved method of blade diagnostic testing for a rotating machine. The present invention is designed to address this need.